Conventionally, an arc welding robot, a sealing robot and the like are equipped with a laser sensor for detecting a welding position or a sealing position etc.
This laser sensor is comprised as shown in FIG. 1. In FIG. 1, a detecting unit 10 includes a laser oscillator 11, a swing mirror (i.e. galvanometer) 12 that causes a laser beam emitted from the laser oscillator 11 to scan, and an optical system 13 that converges a reflection beam reflected back on an objective so as to form an image on a light receiving element 14. A control unit 20 includes a laser actuating unit 21 for actuating the laser oscillator 11 to generate a laser beam, a mirror scanning unit 22 that causes the swing mirror 12 to pivot about its axis, and a signal detecting unit 23 for detecting a position of a received beam according to the position detected by the light receiving element 14.
The laser oscillator 11 is actuated by the laser actuating unit 21 to emit a laser beam. The mirror scanning unit 22 cooperates with the laser actuating unit 21 to cause the swing mirror 12 to perform a scanning so that the laser beam emitted from the laser oscillator 11 is directed toward a predetermined point on the object 30. The laser beam reflected back on the object 30 is converged by the optical system 13 to form an image on the light receiving element 14 in accordance with the reflected position on the object 30. This light receiving element is normally constituted of a PSD (Position Sensitive Detector) of non-split & integration type element, or a CCD (Charged Coupled Device) of split type element.
In the case where a PSD is used as a light receiving element, a beam received on a light receiving surface as shown in FIG. 2, i.e., an image of a reflected beam, is converted into a photoelectric current, and then outputted from electrodes on both sides of the element. A light receiving position x.sub.a is determined based on the values of two electric currents outputted from the electrodes on both sides of the light receiving element.
To be more specific, given that a distance from the center of the light receiving element to the respective electrode is L; a distance from the center of the light receiving element to the light receiving position is x.sub.a ; and further, the two electric current values are I.sub.1 and I.sub.2, the distance x.sub.a can be obtained by the following equation 1. EQU x.sub.a =L.multidot.(I.sub.2 -I.sub.1)/(I.sub.2 +I.sub.1) (1)
On the basis of thus obtained position x.sub.a on the light receiving element, a position of the object 30 with respect to the sensor is calculated as will be described later.
In the case where a CCD is used as the light receiving element, a beam received on a light receiving surface, i.e., an image of a reflected beam, is converted into a photoelectron and then stored into its cell. Electric charges stored in respective cells are outputted one after another from an outermost end at predetermined intervals. In the case of a CCD, the larger the quantity of the light received by a cell, the larger the amount of the electric charge stored by the cell. Accordingly, a position that has received the largest quantity of light reflected can be identified by detecting the position of the cell that gives the largest output. Thus, on the basis of such a position, the position of the object 30 with respect to the sensor can be calculated.
FIG. 3 is a view illustrating a method for determining a coordinate position (X, Y) of the object 30 with respect to the sensor on the basis of the position x.sub.a detected by the light receiving element 14. In this method, assume that a sensor origin (0, 0) is located on a straight line passing the center of the optical system 13 and the center of the light receiving element 14 (this straight line is hereinafter referred to as Y-axis, whereas a straight line perpendicular to the Y-axis is referred to as X-axis). Further assume a distance between the origin of the sensor and the center of the optical system 13 is defined as L.sub.1 ; a distance between the center of the optical system and the center of the light receiving element 14 is defined as L.sub.2 ; an X-axis distance between the origin of the sensor and the pivot center of swing mirror 14 is defined as D; a Y-axis distance between the sensor origin and the pivot center of swing mirror is defined as L.sub.o ; a reflection angle of laser beam that is reflected at the swing mirror 12 toward the object 30 is defined as .theta. with respect to the Y-axis; and a distance of light receiving position on the light receiving element 14 is defined as x.sub.a. Based on these assumptions the coordinate position (X, Y) at which the laser beam is received and reflected can be determined by calculating the following equations. EQU X=x.sub.a .multidot.[(L.sub.1 -L.sub.o).multidot.tan .theta.+D]/(x.sub.a +L.sub.2 .multidot.tan .theta.) (2) EQU Y=[L.sub.1 .multidot.x.sub.a +L.sub.2 .multidot.(L.sub.o .multidot.tan .theta.-D)]/(x.sub.a +L.sub.2 .multidot.tan .theta.) (3)
In the case where a PSD is used as a light receiving element, as the PSD is a non-split & integration type element, such a light receiving element has very high detecting resolution. However, a PSD is disadvantageous in that the entire quantity of the light received on the light receiving surface is converted into photoelectric current, and thus the light receiving surface of the PSD tends to pick up not only a reflection beam required to be detected but also, a noise light occurring such as an arc light occurring in case of arc welding or a secondary reflection beam.
Accordingly, such a noise light is also converted, together with the required reflection beam, into a photoelectric current, and outputted from the electrodes provided at both ends of the light receiving element. As a result, the thus obtained photoelectric current adversely affects the detection of beam position in such a manner that the detected beam position, i.e., an incident position of the reflected beam, is offset from an actual position. Then, such a deviation of the detected beam position results in an overall deviation of the object position obtained through the calculation by above second and third equations, thereby deteriorating a detecting accuracy of the sensor.
For example, as shown in FIG. 4, a beam B.sub.o having been reflected on the swing mirror 12 is then reflected back at the object 30 as a primary reflection beam B.sub.1 to form an image on the light receiving element 14. On the other hand, part of the beam reflected at the object 30 is further reflected at another portion of the object as a secondary reflection beam B.sub.2 to form another image on the light receiving element 14.
Accordingly, the light receiving element 14 receives both the primary and secondary reflection beams. Thus, photoelectric currents outputted from both ends of the light receiving element 14 are affected by these primary and secondary reflection beams in such a manner that the output position x.sub.a is obtained as a position based on the composition of the primary reflection beam and the secondary reflection beam. Hence, the position of the object 30 calculated based on thus wrongly obtained output position x.sub.a is offset from an actual position, as indicated by 30' in FIG. 4.
In the case where a CCD is used as the light receiving element 14, since each of cells constituting the CCD can perform a photoelectrical conversion independently, it is possible to detect the noise light or the secondary reflection beam separately from the primary reflection beam. Therefore, the CCD is more advantageous compared with the above-described PSD in that it is free from the adverse effect of secondary reflection beam and the like.
However, this CCD has another disadvantage. That is, each cell of the CCD has to occupy a relatively large area required for receiving significant light quantity on it. Therefore, a distance between two adjacent cells is inherently large. This may result in a deterioration in a sensor resolution. In order to improve sensor resolution, it may be possible to provide a narrow cell which is capable of receiving a satisfactory light quantity. But it is not desirable in that, if it is desired to fabricate it, production costs of the element become expensive. Alternately, in order to improve sensor resolution, the number of cells constituting the CCD may be increased. However, such an element will unnecessarily increase the overall size of the element and, therefore, will not be practically used.
Furthermore, as the laser beam has coherency, an output waveform of the CCD is likely to become uneven. Therefore, the CCD has a problem such that the position of the cell to produce the largest output tends to change, and so it becomes impossible to correctly detect the beam receiving position.